i have vibration in my camaro though i got a 1LT automatic it does it at 45 to 55 mph i think its the transmission it gos too low in the rpm band and down downshift i brought it back to my dealer acouple weeks back (like 2) they ran a full test on it looked up recalls nothing

though i did test drive a white 1LT (mirror image of my car) and noticed it on that but not as bad as on mine maybe its my 20 inch wheels

though when i run it in manual mode (tap shift) it does it too if i dont downshift so thats why i think that

another theary i have is maybe it trys to keep it in the higher gears ie 4 5 6 to save gas? i dont know its weird if i notice it geting worse im going back to the dealership